Morphology and phylogeny of ‘Osteolepiform’ fishes : new insights from Australian fossils

2017-01-16T22:47:26Z (GMT) by Holland, Timothy
New Australian ‘osteolepidid’ tetrapodomorph fish material is described, constituting members of the paraphyletic ‘Osteolepiformes’. This early Late Devonian material (ranging between Givetian and Frasnian in age), adds significant information regarding the anatomy and phylogenetic relationships of ‘osteolepidid’ tetrapodomorph fishes. Previously undescribed elements from Marsdenichthys longioccipitus of the Avon River Group, Mt. Howitt, Victoria includes anatomy of the snout, cheeck and palate. New autapomorphies include a bar-like maxilla being approximately equal in depth to the dentary, and a rectangular anterior termination of the lacrimal. Palatal morphology includes several plesiomorphic characters, including the absence of a posterior process on the vomer, a comparable length between the dermopalatine and ectopterygoid, and a relatively broad parasphenoid. Previous suggestions of an affinity between Marsdenichthys and tristichopterids are unfounded. The scales of Marsdenichthys bear concentric rings on the external surface, similar to those of the megalichthyid Rhizodopsis. No megalichthyid synapomorphies are described from Marsdenichthys. A new tetrapodomorph fish Owensia chooi is described from a near-complete ethmosphenoid unit from the Kevington Creek Formation, Owens Creek, South Blue Range, Victoria. Several autapomorphies are established including an anteroposteriorly directed basipterygoid process, an optic nerve foramen positioned dorsal to the basipterygoid process, and an oculomotor nerve foramen located posterodorsal to the basipterygoid process. Owensia shares several features with cosmine-covered members of the Canowindridae, including a broad, dorsoventrally flat, ethmosphenoid unit lacking discernable bone sutures. The internal braincase anatomy of Gogonasus andrewsae from the Gogo Formation, Paddy’s Valley, Kimberley Region, Western Australia is described using micro computed tomography. Complete ethmosphenoid and otico-occipital material reveals unprecedented details of the cranial cavities and vascular canals in tetrapodomorph fishes. An innervated, median positioned capsule underlying the median postrostral and rostral series of the ethmosphenoid unit suggests electroreceptive function. A suture separating the ethmoid from the sphenoid may indicate an early ontogenetic state, while numerous asymmetrical features of the endocranium could suggest growth abnormalities caused by anoxic environmental stressors. New information is presented on the pectoral girdle and fin endoskeleton of Gogonasus andrewsae, including the first description of the anocleithrum, cleithrum, scapulocoracoid and lepidotrichia. New autapomorphies include a large sensory line canal opening on the anterior lamina of the posttemporal, a square region of cosmine on the supracleithrum, a knob-like process on the scapulocoracoid, a relatively small entepicondyle, and I-beam-shaped lepidotrichia. Numerous poorly ossified regions on the scapulocoracoid and humerus are suggestive of an early ontogenetic stage. The pectoral fin endoskeleton of the megalichthyid Cladarosymblema is revealed through radiography, showing a scythe-like radius terminating distally with that of the intermedium. A phylogenetic analysis of tetrapodomorph fishes places Gogonasus within a poorly supported ‘Osteolepidide’, stemward of tristichopterids, ‘elpistostegids’ and tetrapods, indicating a paraphyletic ‘Osteolepiformes’.